1. Field of the Invention
This invention relates to a magnetooptical recording medium which may be used for optomagnetic memory, magnetic recording and display elements, and more particularly, to a magnetic thin film recording medium capable of being read by the magnetooptical effect.
2. Description of the Prior Art
Heretofore, there have been known various magnetooptical recording medium, for example, polycrystalline thin films such as MnBi, MnCuBi and the like, amorphous thin films such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe and the like, and single crystalline thin films such as GIG and the like.
Among these thin films, the amorphous thin films have been recently regarded as excellent magnetooptical recording mediums since the thin films of a large area can be produced at about room temperature, signals can be written with a small light-thermal energy at a good writing efficiency, and the written signals can be read out at a good S/N ratio at a high read-out efficiency.
However, these amorphous thin films suffer from various drawbacks. For example, GdFe has a small coercive force and the recorded information is not stable.
In the case of GdFe or GdCo, the writing is effected utilizing the magnetic compensation temperature and therefore, there is a drawback that the film composition should be strictly controlled upon forming the film so as to make the writing efficiency uniform.
In the case of TbFe, DyFe or TbDyFe, a Curie temperature (Tc) writing is effected, and therefore, it is not necessary to control so strictly the film composition, but there is a drawback that the Curie temperature is as low as 100.degree. C. or less so that a light having a strong power can not be used upon reading the signal.
Further, it is easy to produce a medium of a large area of the amorphous thin film, but in general, the performance index corresponding to reading efficiency for reading the signal is small as compared with MnBi polycrystalline thin films and GIG signal crystalline thin films and there is not obtained a sufficient S/N ratio.
British Patent Publication No, 2,071,696 proposes amorphous ternary alloy thin films composed of Gd-Tb-Fe which have a large magnetooptical constant of amorphous thin films such as the angle of Kerr rotation and the like to effect reading at a good S/N ratio.
The angle of Kerr rotation and Curie temperature of conventional magnetooptical thin films are shown in Table 1 below.
TABLE 1 ______________________________________ Angle of Kerr rotation Curie temperature Material (degree) (.degree.C.) ______________________________________ DyFe 0.12 62 TbFe 0.18 91 GdFe 0.24 Compensation*.sup.1 temperature (T comp.) GdCo 0.2 Compensation*.sup.1 temperature (T comp.) TbDyFe 0.20 75 GdDyFe 0.24 120 GdTbFe 0.27 150 measuring wavelength 6328.ANG. ______________________________________ *.sup.1 Compensation temperature writing
The angle of Kerr rotation is defined as shown below. A linearly polarized light incident on a magnetooptical thin film is reflected as an elliptically polarized light. The angle of Kerr rotation is the angle formed by the polarized plane of the incident light and the plane formed with and containing major axes of the resulting elliptical polarized light.
As is clear from Table 1, the largest angle of Kerr rotation is 0.27 degree for GdTbFe. For reading at a stable S/N ratio, at least 0.2 degree or more of the angle of Kerr rotation is desirable. It goes without saying that the larger magnetooptical constants such as the angle of Kerr rotation and the like, the better the S/N ratio upon reading.
The lower the Curie temperature, the higher the efficiency of writing, but the signal written in is disturbed by the ambient temperature and the read-out light at a low Curie point. Therefore, the magnetic transformation temperature is preferably about 100.degree. C.-200.degree. C. taking the practical use situation into consideration.
On the other hand, the alloys in Table 1 show ferrimagnetism so that there are compensation compositions. The coercive force is very large near the compensation composition region.
In general, the angle of Kerr rotation effecting the S/N ratio and the coercive force influencing the stability of recorded magnetic domains change independently from each other with respect to the alloy composition, and therefore, an alloy composition having a large angle of Kerr rotation does not always exhibit a large enough coercive force.
Since an alloy composition having a very large coercive force needs a very large magnetic field for magnetization or demagnetization when it is used for a recording medium, such an alloy composition is not desirable from the practical point of view even if the angle of Kerr rotation is large.